A fast dose modulation in exposure tubes, in particular X-ray exposure tubes, is desirable to minimise a patient dose in a computer tomography (CT). The desired speed of modulation increases with an increased gantry speed in order to enable a faster control of the photon flux. While maintaining a high photon flux in those phases of a computer tomography scan, in which phases diagnostic information has to be gained with a high definition or where the penetration through the object is poor, it is desirable to cut back on photon flux in other phases.
US 2005/0163281 A1 describes an X-ray tube which includes a device for at least substantially protecting an object to be examined against the incidence of undesirable X-rays, which can be produced noticeably by the decay of a residual or surplus charge present in a high voltage circuit after an X-ray exposure. US 2005/0163281 A1 describes a device for deflecting and/or defocusing the electron beam produced by the residual and/or surplus charge in such a manner that at least it is not incident to a significant extent on a region of an anode where from X-rays excited thereby are directed towards an object to be examined, namely to an exterior radiation collector.
In some applications, the flux should not cease to zero, but remain at a certain level for a while, and the maximum focal spot size should be maintained, at least not exceeded. If the transient is fast compared to the period of one computer tomography view, typically some hundred microseconds, pulse with modulation may become possible to control the overall photon flux very quickly. In present applications, the photon flux is either controlled by switching the high voltage on and off with a transition time of about half a millisecond, or by driving the filament temperature of the tube up and down within some hundred milliseconds. Thus, either the modulation is not maintaining a certain minimum level, and/or it is too slow or the focal spot is unacceptably distorted.